Insulin and IGF-I receptors are the products of distinct genes and mediate different functions. The insulin receptor regulates metabolic functions whereas the IGF-I receptor regulates mitogenic functions. Both receptors possess tyrosine kinase activity, but exhibit distinct tissue specifications, development regulation and certain structural differences. Furthermore, in tissues such as the brain, the insulin and IGF-I receptors differ from their counterparts in other tissues. To investigate these differences, we have cloned and sequenced the rat IGF-I receptor cDNA and its proximal promoter region. These studies as well as investigations of IGF-I receptor gene expression in development and certain pathological states will contribute to our understanding of the precise mechanisms and functions of the IGF-I receptor. In addition, cDNA probes for various glucose transport proteins are used to study the role of insulin and IGF-I in the metabolic functions of brain tissue which express large numbers of IGF-I receptors.